1. Field of the Invention
The present invention relates to a secondary cell, and more particularly, to a gel type polymer solid electrolyte, a method for manufacturing the polymer solid electrolyte, and a lithium secondary cell adopting the polymer solid electrolyte.
2. Description of Related Art
As portable electronic devices having high degree functions such as camcorders, cellular phones, laptops, etc. become smaller and lighter, research has been conducted to develop a storage cell having high energy density that can be used as a driving source for these devices. In particular, a lithium secondary cell has high energy density per weight, as much as three times that of a conventional lead storage cell, nickel-hydrogen cell or nickel-zinc cell, as well as having the capability of being rapidly recharged. Consequently, significant research has been conducted to develop a lithium secondary cell for use in such products.
As an cathode active material for the lithium secondary cell, lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2) or lithium manganese oxide (LiMn.sub.2 O.sub.4) typically are used. Also, as a anode active material, lithium metal, a lithium metal alloy or carbon typically is used.
In addition, a liquid electrolyte generally is used as an electrolyte for the lithium secondary cell. However, if a cell adopting the liquid electrolyte is used for an electric device, the device may be damaged due to leakage of electrolyte solution and the inside of the cell may become dry due to volatilization of solvent. Accordingly, shorts can occur between electrodes. In order to solve these problems, usage of solid electrolyte instead of the liquid electrolyte has been proposed. Generally, solid electrolytes do not leak electrolyte solution and are easily processed. Thus, research into the use of solid electrolytes has been actively performed. There is a particularly high interest in a polymer solid electrolyte. Polymer solid electrolytes can be classified into: (i) a solid type without an organic electrolyte solution; and (ii) a gel type including organic electrolyte solution.
Known polymer solid electrolyte typically include a polymer solid electrolyte produced by combining an alkyleneoxide polymer having the end modified with an acryloyl group, a low molecular weight alkyleneoxide polymer whose both ends are alkyleterified, polyvinyl chloride and electrolyte salt, and a polymer solid electrolyte produced by combining an alkylenoxide polymer having the ends modified with acryloyl group, inorganic ion salt and organic solvent such as propylene carbonate. However, these polymer solid electrolytes are disadvantageous in electrochemical properties, particular, in ion conductivities, and in a mechanical strength.
Other well known polymer solid electrolytes include a polymer solid electrolyte containing polyvinylidene fluoride (PVDF) as a polymer matrix, lithium perchlorate (LiClO.sub.4) as a support electrolyte and a propylene carbonate as a solvent. However, the electrolyte has insufficient conductivity of 10.sup.-5 .about.10.sup.-3 S/cm at room temperature, so that it is difficult to apply the electrolyte to a cell for practical use.